The basic principal of a hexapod CMM 10 is presented on the bases of FIG. 1. In principal a hexapod CMM comprises a base structure 12 and a moveable structure 14 that are spaced apart by usually six telescopically extendable legs 16 (16a, 16b, 16c, 16d, 16e, 16f). Usually the six telescopically extendable legs 16 (16a, 16b, 16c, 16d, 16e, 16f) are connected with their bottom ends to the base structure 12 using spherical joints 28 (28a, 28b, 28c, 28d, 28e, 28f). They are also connected with their top ends to the moveable structure 14 using spherical joints 26 (26a, 26b, 26c, 26d, 26e, 26f). The spherical joints 26, 28 may be build as a ball joint, cardan joint or Hooke's joint, a flexural mechanism etc. well known by skilled persons. (In this application the term “spherical joint” is used as a general term for all those different joints allowing more or less arbitrary spherical rotation like: flexural mechanism, ball joints, cardan joints or Hooke's joints etc.). Usually, as it is also shown in FIG. 1, each extendable leg 16 is connected with its bottom end pairwise—namely 16a and 16b, 16c and 16d, 16e and 16f—with the adjacent extendable leg arranged at its one side, while the same extendable leg 16 is connected with its top end pairwise with the adjacent extendable leg of its other side—namely 16a and 16c, 16b and 16f, 16d and 16e—, so that the leg pairs form triangles with a rim of the base structure 12 and a rim of the moveable structure 14, respectively.
Each telescopically extendable leg 16 comprises a drive means (motor and according mechanism, not shown), at least one sliding linear member 23 configured to slide along at least one stationary linear member 22—the stationary linear member is often a hollow member accommodating the sliding member—and an according number of linear encoders 24 measuring the relative displacement of the at least one linear sliding member 23 relative to the stationary linear member 22 or relative to further linear sliding members (not shown). The length of each extendable leg 16 is controlled by an associated controller (not shown) for each leg or a central controller 38 using the information generated by the linear encoder(s) 24 to control the drive means in a servo loop. A program for controlling the movement of the legs 16 and by this the movement of the movable structure 14 may be stored in an internal memory 39 connected to the controller. In preferred embodiments the controller 38 further includes an analysing unit 32.
Usually the linear members 22, 23 of a leg 16—also called leg-members—are made of a light material as aluminum, an aluminum alloy or plastics and in particular reinforced plastics comprising glass-fibres and/or carbon fibres. Also the movable structure 14 is usually made of a light but rigid material like the materials used for the leg-members, wherein the base structure 12, when it forms the bottom of the CMM to stand on, is often made of a more heavy material.
In use, an object 20—also called a workpiece 20—that is to be measured, is placed on the base structure 12 of the hexapod CMM 10. Optionally fixation means 18, like brackets, can be provided in order to fix the workpiece 20 to the base structure 12. Altering the length of the various legs 16 allows the moveable structure 14, and hence a probe 30 fixed to the movable structure 14, to be moved relative to the base structure 12 and the workpiece 20 with 6 degrees of freedom (6 DOF): 3 degrees of freedom regarding the lateral movement in the Cartesian-directions x, y, z and further 3 degrees of freedom regarding rotational movement around these Cartesian x-, y-, z-axis resulting in yawing, rolling, pitching. As a result the form of an workpiece 20 placed on the base structure 12 of such a hexapod CMM 10 can easily be measured by the probe 30 mounted to the movable structure 14 of said hexapod CMM 10 from different perspectives. Thereby, the moveable structure 14 and the extendable legs 16—including the movement options of the legs 16—define a working volume, the probe is moveable within said working volume. Results of the measurement of the probe 30 might be analysed in an internal control unit 38 and/or using an external control unit 36; wherein at least one of the controllers 38, 36 comprises an analysing unit 32. Data transfer within the CMM, i.e. between the linear encoders and the controller(s) or the memory 39 and the controller(s) or the probe 30 and the internal or external control unit, as well as to external units in general might be operated by wire 35 or wireless 37, i.e. by bluetooth, infra red, wlan etc. A further example of the same type of a hexapod CMM is given in U.S. Pat. No. 7,841,097 B2.
A slightly different example of a Non Cartesian CMM 10′ is shown in FIG. 2. Despite of all differences (see below), the NCCMM 10′ shown in FIG. 2 is based on the principals of the hexapod CMM described above for FIG. 1. It is therefore called a hexapod-like CMM. However, in this embodiment a bottom platform 9 carries the workpiece 20 and supports piles 8, which piles 8 (adumbrated by dashed lines) support the base structure 12 on their top in a fixed position. The movable structure 14 is arranged hanging from this top arranged base structure 12 on three (instead of six) extendable legs 16 (16a, 16b, 16c). A probe 30 is fixed to the hanging movable platform 14 directed towards the bottom platform 9 carrying the workpiece 20. The movable structure 14 with the probe 30 is movable relative to the bottom platform 9 and to the base structure 12, respectively, with 6 degrees of freedom (see above), by extension and contraction of the legs 16, so that the workpiece 20 on the bottom platform 9 can be measured from all sides and from all perspectives.
Independent on the concrete design of the hexapod CMM or hexapod-like CMM the telescopically extendable legs are formed as struts, most often in the form of an outer hollow tubular portion and at least one inner tubular portion, wherein each of the at least one inner tubular portions is slidable within the next outer hollow tubular portion. However, instead of hollow elements as leg-members it is also possible to design the struts and leg-members, respectively, in form of i.e. tongue-and-groove, dovetail guide, u-sections or a u-section and a t-section slidable against each other or sections with other cross sections suitable for sliding relative to each other.
The drive means may comprise any arrangement that introduces relative axial motion between a first leg-member and a second leg-member or even further leg-members and thus allows extension and contraction of the leg. For example, the drive means may be hydraulic, or may comprise a screw jack, a direct drive mechanism or an electronic drive arrangement including e.g. a friction drive like it is described in U.S. Pat. No. 5,604,593. Each of the six extendable legs of the hexapod CMM of U.S. Pat. No. 5,604,593 comprises in each case a tubular housing and a tubular ram member, which tubular ram member slides telescopically within the tubular housing driven by means of a friction drive. This friction drive comprises a motorised drive roller, and an opposing reaction roller, both of the rollers engage with the tubular ram member such that rotation of the motorised drive roller 54 extends the ram member from the tubular housing or retracts the ram member into the tubular housing.
While changing the length of the telescopically extendable legs the noise level of the spherical joints connecting the legs with the movable structure and with the base structure are often high. Further, due to lubrication misbalances friction effects and stick slip effects might be present. U.S. Pat. No. 5,604,593 suggests to build the spherical joints in form of air ball bearings, in order to avoid these disadvantages. Each of the six extendable legs is connected to the base structure and to the movable structure by an air ball bearing having a universally rotatable ball. In each case two of such air ball bearings are embedded in a connecting element. Thus, there are three connecting elements connecting the movable structure with the tubular ram members of the six legs and three further connecting elements connecting the base structure with the six tubular housings of the six legs. However, more details of the structure of the air ball bearings, i.e. the air supply system, the construction of the bearing housing etc. are not disclosed in U.S. Pat. No. 5,604,593.
But U.S. Pat. No. 5,604,593 does not only try to avoid the disadvantages of common spherical joints but also suggests to reduce the influence of load and vibrations on the measurement. For this purpose U.S. Pat. No. 5,604,593 suggests to measure the relative displacement of the moveable structure to the base structure not any more by a linear encoder integrated in the extendable legs, but to measure it by six tracking interferometers each comprising a light source, a retro reflector and a detection unit for detecting the light reflected by the retro reflector. The light sources and the detection unit are placed together at the base side, wherein the retro reflector is mounted linear opposite at the movable structure. The interferometers are mounted either within the hollow rams or beside the spherical bearings connecting the legs to the base structure and to the moveable structure. Each tracking interferometer tracks and measures the distance to its corresponding retro reflector and a computer control is enabled to calculate from the six linear measurements the obtained position and orientation of the moveable structure including position and orientation of the probe fixed to the moveable structure.
U.S. Pat. No. 7,841,097 B2 suggests a different strategy to eliminate the negative influences of load impacts, deflection by weight, inertia effects and vibrations during movement on the preciseness of the measurements. The hexapod NCCMM of U.S. Pat. No. 7,841,097 B2 discloses a hexapod CMM with a hexapod double structure: a load bearing hexapod structure including motor driven struts and a load bearing base structure and a load bearing movable platform. Further, and parallel to this load bearing hexapod structure a hexapod metrology structure is provided coupled with the load bearing structure. The metrology hexapod structure comprises a metrology base structure, a metrology moveable platform carrying the probe and extendable legs, expansion and contraction of these legs are operated by the motor driven struts of the load bearing parallel hexapod structure. According to the discloser the metrology moveable platform is connected to the parallel load bearing moveable platform, the metrology base structure is connected to load bearing base structure and the metrology legs are connected to the load bearing, motor driven struts in a way that no or very little load and no or very little vibrations are transmitted from the load bearing structure to the metrology structure. As set above, the metrology extendable legs are actuated in their extension and contraction by the load bearing extendable legs of the load bearing hexapod structure. The metrology extendable legs are provided with linear encoders in order to measure the relative displacement of their extendable and contractionable parts, respectively, without any influence of load deflection or motor/movement vibrations, which is carried solely by the load bearing legs. However the hexapod double structure is very expensive.
The proposed solutions of U.S. Pat. Nos. 7,841,097 B2 and 5,604,593 to avoid a high noise level, friction and stick-slip-effects connected with the spherical joints as well as a negative influence by inertia effects, vibrations, deflection by weight, are very complicated, expensive and lead to high maintenance and service costs and moreover may reduce the possible operation speed of the NCCMM.